The invention relates to a method as well as to an apparatus for manufacturing a punch riveted joint.
In the automotive industry, in particular, riveted joints are increasingly gaining in importance because they may form an alternative to a weld joint. For material pairings which are difficult to weld, riveted joints are a suitable alternative. By means of punch riveting, the parts to be joined are connected without pre-punching. The pre-punching of the joint parts which is necessary with conventional riveting is replaced by a corresponding cutting operation of the punch rivet.
When punch riveting using a semitubular rivet, the components to be joined are placed onto a die. Said components are fixed on the die by means of a holding-down device. The semitubular rivet cuts through the layer directed towards the semitubular rivet. Then the semitubular rivet is plastically deformed in the bottom layer or layers so as to form a closing head. The configuration of the closing head is determined substantially by the shape of a die. The material punched out of the top layer and, occasionally, out of further layers fills the hollow rivet shank of the semitubular rivet and is held captive therein. The semitubular rivet is then upset so that, on the one hand, a further formation of the closing head is effected and, on the other hand, the head of the semitubular rivet terminates substantially flush with the top layer.
From the article by L. Budde and W. Lappe xe2x80x9cRiveting without pre-punchingxe2x80x94punch riveting has a promising future in sheet metal workingxe2x80x9d, which appeared in the periodical xe2x80x9cStrips Sheets Pipesxe2x80x9d 5-1991, pages 95-100, it is known that crucially important factors for a high-quality punch riveted joint by means of a semitubular rivet are, on the one hand, trouble-free fixing of the positioned components so that the relative position of the components is not varied as a result of an after flow movement during punching and, on the other hand, joint-specific selection of a suitable punch rivet in terms of material-specific composition and its rivet geometry.
To ensure that an after flow movement during punching does not occur, it is known from WO 94/14554 for the clamping force of the holding-down device used to fix the components on the die to increase considerably towards the end of the punch riveting operation. A further object to be achieved by said refinement of the method of manufacturing a punch riveted joint is that no annular indentations or distortions arise in the region of the head of the punch rivet.
For effecting the method, WO 94/14554 proposes an apparatus which comprises a die, a holding-down device and a punch. At least two layers of components are clampable between the holding-down device and the die. The holding-down device is movable by means of a first piston/cylinder unit. A second piston/cylinder unit is provided as a drive unit for the punch. The first and the second piston/cylinder unit are operable independently of one another.
A similar refinement of an apparatus for manufacturing a punch riveted joint is known from WO 93/24258. Said apparatus also comprises a piston/cylinder unit, by means of which the required clamping force of the holding-down device may be generated. Differing clamping forces may be generated by suitable pressure control. The apparatus according to WO 93/24258 comprises a second piston/cylinder unit as a drive unit of the punch.
The mechanical properties of a punch riveted joint, in particular the strength characteristics of the punch riveted joint, are dependent upon the formation of the closing head. The strength characteristics are also influenced by the configuration of the region of the components between the head and the closing head of the punch rivet.
Against said background, the object of the present invention is to develop the known method of manufacturing a punch riveted joint in such a way as to achieve, on the one hand, material-sparing manufacture of a punch riveted joint and, on the other hand, a punch riveted joint of greater strength. A further object of the invention is to indicate an apparatus for manufacturing a punch riveted joint, which is suitable for effecting material-sparing manufacture of a punch riveted joint. High strength values of the punch riveted joint manufactured by means of the apparatus are also to be achieved by virtue of the apparatus.
The effect achieved by the method according to the invention for manufacturing a punch riveted joint is that during a punch riveting operation a clamping force exerted by the holding-down device upon the at least two layers may be varied in accordance with a preset characteristic. A variation of the clamping force exerted by the holding-down device upon the at least two layers is achieved in that the holding-down device is connectable by a coupling unit to the punch in such a way that during a punch riveting operation a coupling of the holding-down device to the punch may be varied between a substantially rigid coupling state and an uncoupled state. The effect achieved by said process management is that in the rigid coupling state the holding-down device is movable synchronously with the punch, in particular in the direction of the die. In the uncoupled state a movement of the punch may be effected while maintaining or varying the clamping force exerted by the holding-down device upon the components.
By virtue of the fact that the clamping force exerted by the holding-down device upon the components is variable on account of said clamping force being dependent upon the movement of the punch, the effect is also achieved whereby the requisite clamping force for individual portions of the punch riveting operation is always produced. The amount by which the clamping force is varied is dependent upon the coupling state. In particular, it is proposed that the coupling state is varied as a function of the punch travel or a parameter derived from the punch travel.
Depending upon material pairings as well as upon mechanical properties of the materials of the layers, it may be advantageous for the layers to be pressed initially with a reduced clamping force against the die. Later, the layers may be pressed with axe2x80x94compared to the reduced clamping forcexe2x80x94varying clamping force against the die. In particular, the clamping force should be increased during the upsetting operation. This has the advantage that, in the event of formation of a bead at least partially surrounding the head of the tubular rivet, said bead is upset by the holding-down device in such a way that the head of the tubular rivet terminates flush with the layer. The clamping force may in said case also be much higher than the customary clamping force.
The variation of the coupling state as a function of the punch travel may be effected continuously or discontinuously. Continuous variation of the coupling state is preferred because it does not lead to any abrupt alteration of the clamping force. Alternatively, given discontinuous variation of the coupling state, the jumps in the clamping force may be selected in such a way that no pronounced abrupt alteration of the process parameters occurs.
By virtue of a rigid coupling of the holding-down device to the punch, the holding-down device executes the same movement as the punch. The holding-down device exerts substantially the same force as the punch. In accordance with a preset clamping force characteristic, the holding-down device may be uncoupled from the punch through control of the coupling unit. The further exertion of clamping force by the holding-down device is controlled by means of the coupling unit. When after a preset punch travel the holding-down device is to exert a higher force upon the layers, the holding-down device may be re-coupled to the punch by the coupling unit.
In particular, it is proposed that the holding-down device is moved with the punch relative to the layers to be joined in such a way that the holding-down device presses the layers with a preset clamping force against the die. The holding-device is uncoupled from the punch and the punch is moved further for cutting through at least the layer directed towards the semitubular rivet, forming a closing head and upsetting the semitubular rivet, the holding device optionally being coupled to the punch after a preset punch travel.
A variation of the clamping force as a function of the punch travel is effected preferably in accordance with a preset characteristic of the clamping force as a function of the punch travel. The preset clamping force/punch travel characteristic is in said case a setpoint characteristic, which is preferably compared continuously with the actual characteristic of the clamping force as a function of the punch travel. To said end, it is proposed that a punch travel is measured and it or a parameter derived from the punch travel is compared with a preset setpoint value. Preferably, as a function of the comparison result the holding-down device is uncoupled from the punch or coupled to the punch.
By virtue of the process management according to the invention a gentle plastic deformation of the punch rivet in at least one layer is also guaranteed without excessive weakening of said layer or layers. An excessive weakening of the layer or layers may lead to an unintentional punching-through of said layer or layers. A punching-through of the layer or layers would, depending upon the material pairings, result in corrosion. By virtue of the method according to the invention said problem does not however arise.
According to an advantageous development of the method, it is proposed that the coupling state is varied as a function of the material properties, in particular upon the strength, of at least one layer. The idea behind said advantageous development is that the layers may present differing flow and/or deformation properties. In particular, it is proposed that a clamping force/punch travel characteristic is varied as a function of the properties of the pairing of the layers of different materials. When, for example, a layer of aluminium or an aluminium alloy is to be joined by means of a punch rivet to a second layer of aluminium or an aluminium alloy, then the coupling state is varied in a different manner to a join between aluminium and steel. In so doing, it is also possible to take into account that the coupling state may also depend upon which material properties are presented by the layer to be punched through and/or the layer to be deformed.
For a further improvement of the formation of a punch riveted joint, it is proposed that the coupling state is varied as a function of a thickness of at least one layer, in particular of the layer directed towards the punch rivet. This is to take account of the fact that the flow properties of the layer(s) are dependent also upon the thickness of the layer(s). With a relatively thin layer there is more likely to be a risk of the layer being overstressed in the region of the head of the punch rivet, possibly leading to textures or cracks. This may be avoided by allowing a flow movement of said layer towards the semitubular rivet at least during part of the punch riveting operation, the coupling state being varied from a rigid to a less rigid connection between the holding-down device and the punch. For relatively thick layers the clamping force may be selected higher than for layers of a relatively low thickness because thick layers present stress characteristics of a different kind. Overstressing occurs much later in the thick layer than in a thin layer because there is sufficient material for a flow movement into the die and towards the semitubular rivet.
According to a further advantageous refinement of the method, it is proposed that the coupling state is varied as a function of the material properties of the punch rivet. In particular, the hardness of the punch rivet is proposed as a criterion for varying the coupling state.
The geometry of the punch rivet, in particular the configuration of the free end face of the punch rivet, as well as of the recess of the semitubular rivet also plays a role in the configuration of the riveted joint. The geometry of the punch rivet influences the punching and deformation operation. It is therefore proposed that the coupling state is varied as a function of a geometry of the punch rivet. For the geometry of the punch rivet, individual geometric quantities of the punch rivet may also be used as a criterion. Here, it is possible to use, for example, the cross section of the punch rivet and the length of the punch rivet as factors. The further geometric quantities of a punch rivet may also be used as criteria for the selection of a suitable coupling state characteristic.
In particular, the coupling state may at least during at least one part of an upsetting operation be reduced to such an extent as to enable a movement of at least one layer in a direction substantially at right angles to the semitubular rivet. The effect thereby achieved is that a material flow occurs during the formation of the closing head so that the layers in the region of a punch riveted joint are subject to a lower mechanical stress, in particular to a tensile load.
As a further criterion for the process management, a punch force of the punch may be determined and compared with a preset setpoint value, the holding-down device preferably as a function of the punch force and/or the punch travel being uncoupled from the punch or coupled to the punch. Additionally or alternatively, during the punch riveting operation the clamping force of the holding-down device may be determined and compared with a preset setpoint value. On the basis of the comparison, the clamping force may be varied as a function of the punch travel.
A setpoint characteristic curve of a clamping force/punch travel characteristic may be determined on the basis of experiments. Here, it has first to be assumed that such a setpoint characteristic curve is valid for the specific punch riveted joint formed during the experiment. For punch riveted joints which vary in terms of the material properties of the layers, their geometry, etc., corresponding further setpoint characteristic curves have to be determined. In a first approximation, when determining a setpoint characteristic curve for a preset punch riveting task, it is possible by extrapolation of the known setpoint characteristic curves to determine an assumed setpoint characteristic curve which may serve as a basis for the comparison with the clamping force/punch travel characteristic.
In view of the fact that the layers may within the manufacturing tolerances differ in thickness, it is proposed that the holding-down device is coupled to the punch as a function of a punch force. A coupling of the holding-down device to the punch may also be effected when the punch force and the travel of the punch lie within a specific tolerance zone.
According to a further advantageous refinement of the method, it is proposed that a clamping force of the holding-down device is measured directly or indirectly. The holding-down device may as a function of the clamping force and/or the punch force and/or the travel of the punch be uncoupled from the punch or coupled to the punch.
It is particularly advantageous when, in the method according to the invention, the holding-down device comprises a piston and the punch comprises a pressure piston, wherein the piston and the pressure piston are movable relative to one another inside a common chamber, wherein a fluid, preferably a substantially incompressible fluid, is provided between the piston and the pressure piston, wherein the distance of the piston and the pressure piston relative to one another is varied substantially by means of the fluid, in particular the fluid volume. The chamber with the fluid as well as the piston and the pressure piston form the coupling unit. By varying the volume or pressure of the fluid between the piston and the pressure piston, the coupling state between holding-down device and punch may be varied. When the fluid volume is constant, in particular given a substantially incompressible fluid, there is a rigid connection between the punch and the holding-down device. Upon a movement of the punch, the holding-down device effects a positional variation corresponding to the punch travel. When the fluid volume is varied during the punch movement and indeed in such a way as to be reduced, then the punch effects a greater punch movement than the holding-down device. The holding-down device may, given a corresponding variation of the fluid volume, be stationary, in which case the force exerted by the holding-down device upon the components may be varied variably or constantly as a function of the fluid volume. In a corresponding manner, the coupling state may be influenced by the fluid pressure.
Preferably at least part of the fluid is directed out of the chamber into a compensating chamber, with the result that the relative position of the piston and pressure piston is varied. The speed at which a positional change of the piston and pressure piston occurs may be influenced by the rate of flow of the fluid out of the chamber. By means of the flow rate, therefore, the rate of change of the clamping force is also influenced. For the formation of a further punch riveted joint, the fluid which has flowed out of the chamber into a reservoir is conveyed out of the compensating chamber back into the chamber, thereby achieving a closed-circuit fluid system.
For the formation of a further punch riveted joint, the fluid which has flowed out of the chamber into a reservoir is conveyed out of the compensating chamber back into the chamber, thereby achieving a closed-circuit fluid system.
According to a further advantageous refinement of the method, it is proposed that the holding-down device is connected to a piston disposed in a double-action cylinder, through which the punch extends. The punch comprises a pressure piston guided in a chamber, which is loadable with a preferably incompressible fluid. The chamber is connected to a reservoir of variable volume. The chamber is connected rigidly to the cylinder and guided displaceably in a housing.
The variation of the fluid pressure is effected preferably as a function of the punch travel and/or the travel of the holding-down device and/or the clamping force and/or the punch force. Preferably, a fluid pressure is determined as a function of the punch travel and the fluid pressure/punch travel characteristic is compared with a preset setpoint characteristic. As a function of the comparison, a pressure control valve fluidically connected to the chamber is activated. The fluid pressure in the chamber is controlled by the pressure control valve. Preferably, the fluid removal is controlled as a function of the clamping force and/or the punch force and/or the travel of the punch and/or the travel of the holding-down device.
When no fluid is removed from the chamber, the pressure piston effects a pressure build-up of the fluid in the chamber, with the result that the fluid exerts a pressure upon the piston of the holding-down device and the holding-down device is moved towards the layers. Once the holding-down device has applied the force required to clamp the layers between the holding-down device and the die, the punch may be moved further, for example, through suitable removal of fluid from the chamber, such that the clamping force between holding-down device and die is maintained. When a more extreme change of volume is effected by fluid removal from the chamber than the change of volume occasioned by the variation of the punch travel, the clamping force of the holding-down device decreases and the punch effects the actual punch riveting operation. This may be effected by no removal or only a lower removal of fluid from the chamber when the layers are to be pressed initially with a reduced clamping force against the die and then with a higher clamping force against the die (compared to the reduced clamping force).
According to a further inventive idea, an apparatus for manufacturing a punch riveted joint is proposed which comprises a die, a holding-down device and a punch. The punch and the holding-down device are movable towards and away from the die. At least two layers of components are clampable between the holding-down device and a die. For connection of the layers, a punch rivet which may be, for example, a semitubular rivet or a solid rivet, is driven by means of the punch at least through the layer of the components which is adjacent to the holding-down device. The apparatus according to the invention is notable for the fact that a coupling unit is provided, which connects the holding-down device and the punch in such a way that during a punch riveting operation a coupling of the holding-down device to the punch may be varied between a substantially rigid coupling state and an uncoupled state. The punch is driven by means of a drive unit. A perceived advantage of said refinement of the apparatus is that the holding-down device is moved as a function of the punch. Also, by virtue of the fact that the holding-down device may be coupled substantially rigidly to the punch, a clamping force is achieved relatively quickly.
According to a further advantageous refinement of the apparatus, it is proposed that the holding-down device comprises a piston head and the punch comprises a pressure piston. The piston and the pressure piston are movable relative to one another in a chamber of the coupling unit which is filled with a preferably incompressible fluid, the fluid pressure in the chamber being variable.
According to a further advantageous refinement of the apparatus, it is proposed that a control device is provided, which is used to activate the coupling unit. The effect achievable by means of the control unit is that during a punch riveting operation, via the coupling unit, a clamping force exerted by the holding-down device upon the at least two layers is varied in accordance with a preset characteristic as a function of at least the punch travel or a parameter derived from the punch travel. The fluid pressure may be controlled by fluid removal from the chamber so that the position of the piston relative to the pressure piston is dependent upon the volume of the removed fluid. When no fluid is removed, there is a substantially rigid connection between the piston and the pressure piston so that the holding-down device is moved synchronously with the punch. Through suitable removal of the fluid the clamping force may be maintained or varied, even though the punch with the pressure piston is moving in the direction of the layers. The fluid removal may also be controlled in such a way that suitable clamping force profiles of the holding-down device may be realised.
The apparatus preferably comprises a compensating chamber, into which at least some of the fluid from the chamber may be fed through at least one line during a punch riveting operation, said fluid being fed out of the compensating chamber and back into the chamber for a new punch riveting operation. At least one line containing a pressure control valve is preferably provided between the compensating chamber and the chamber. The fluid pressure in the chamber may be controlled by the pressure control valve. For automatic control of the apparatus, it is proposed that a pressure sensor for measuring the fluid pressure is provided, which is connected to the control device. The pressure control valve is likewise connected to the control device. Preferably, a fluid pressure is determined as a function of the punch travel and a fluid pressure/punch travel characteristic or a parameter characteristic is determined, which is compared by means of the control device with a preset setpoint characteristic, and as a function of the comparison the pressure valve is activated accordingly. The effect thereby achieved is that the clamping force is controlled substantially via the fluid pressure regulation.
According to a further advantageous refinement of the apparatus, it is proposed that the holding-down device is connected to a piston, which is disposed in a double-action cylinder. The punch extends through the cylinder. The punch comprises a pressure piston guided in a chamber, which is loadable with an incompressible fluid. The chamber is connected to a reservoir of variable volume. The chamber is connected rigidly to the cylinder and guided displaceably in a housing.
To achieve a high-quality punch riveted joint, it is proposed that the apparatus employs a displacement measuring device, by means of which at least a travel of the punch is measured, and as a function of the travel of the punch the holding-down device is uncoupled from the punch or coupled to the punch. The characteristic of the variation of travel of the punch may be used as an indication of the quality of the punch riveted joint to be achieved. A quality check of the punch riveted joint may be carried out already during the formation of the punch riveted joint. The displacement measuring device is preferably disposed on the punch. Alternatively or additionally it is proposed that the apparatus comprises a measuring device, by means of which a punch force of the punch is measured or determined, the holding-down device as a function of the punch force and/or the punch travel being uncoupled from the punch or coupled to the punch.
Alternatively or additionally it is proposed that the apparatus comprises a measuring device, by means of which a punch force of the punch is measured or determined, the holding-down device as a function of the punch force and/or the punch travel being uncoupled from the punch or coupled to the punch.
The measurement of the clamping force of the holding-down device may also be used for quality assurance purposes. In particular, by measuring the travel of the punch, the force of the punch and/or the clamping force of the holding-down device, it is possible to determine the appropriate instant for uncoupling or coupling of the holding-down device to the punch.
The displacement of the holding-down device may also be measured using suitable means. The displacement measurement of the holding-down device may be used as an indication of the thickness of the layers of components to be joined so that the travel of the punch may be determined knowing the thickness of the layers. This is important particularly when the thickness of the layers owing to manufacturing tolerances lies within a tolerance zone. This also prevents, e.g. in the case of a punch riveted joint produced by means of a self-piercing punch rivet, the bottom layer from not being cut through by the punch rivet. By means of the holding-down device the layers may also be pressed towards one another in such a way that they rest against one another before the actual punch riveting operation is initiated.
The driving of the punch may be effected by means of a hydraulic drive unit. Instead of a hydraulic drive unit, it is alternatively proposed that the punch is driven by means of a motor-operated, in particular an electric motor-operated, drive unit. By means of the drive unit, the punch executes a substantially linear motion. Further details and advantages of the method as well as of the apparatus are described with reference to a preferred embodiment.